This invention relates generally to the acquisition and processing of multi-energy computed tomographic data, and more particularly to methods and apparatus for simultaneous spectral-shift and view-aliasing artifact compensations.
In spite of recent advancements in computed tomography (CT) technology, such as faster scanning speeds, larger coverage with multiple detector rows, and thinner slices, energy resolution is still a missing piece. Conventional CT images represent the x-ray attenuation of an object under investigation. Strictly speaking, this definition is not correct, due to the wide x-ray photon energy spectrum from the x-ray source and the lack of energy resolution from detection system used in the conventional CT systems. X-ray attenuation through a given object is not a constant. Rather the X-ray attenuation is strongly dependent on the x-ray photon energy.
The attenuation at a specific point is generally greater for photons of lower energy and causes the energy spectrum to shift as it passes through the body. X-ray beams reaching a particular point inside the body from different directions will typically have different spectra since they passed through materials with different attenuation properties before reaching that point. This creates challenges when attempting to assign a single value to the attenuation at a specific point in the scanned body. This physical phenomenon manifests itself in the image as beam-hardening artifacts such as non-uniformity, shading, and streaks. The visual conspicuity of some of these artifacts can be reduced, but some are much tougher to remove. By filtering the pre-patient beam with an aluminum or copper filter, the lower-energy components of the spectrum can be selectively removed. This lessens the effects of beam hardening effects, but cannot wholly remove these effects. Additionally, there is also a practical limit to the amount of filtration that may be performed. Filtration reduces the total energy flux, which results in an increase in noise. Additionally, the loss of lower energy photons reduces the contrast discrimination.
The literature is rich in beam-hardening correction algorithms that serve to augment filtration. For scans of predominantly soft-tissue anatomy with x-ray spectra and peak kilovoltages typically used for medical CT, beam hardening effects are almost entirely due to Compton scattering. For single energy scanning, a common method to deal with this phenomenon in soft tissue is water calibration, where a uniform water phantom is used to optimize the parameters in a high-order polynomial linearization algorithm. However, photoelectric interactions are also a significant contributor to x-ray attenuation in bone, thus water calibration is not sufficient. Typically, iterative bone correction algorithms are employed where bones are segmented in a first-pass image, then beam hardening from bones are corrected in the second-pass. However, beam hardening from materials others than water and bone, such as metal and contrast agent, become very difficult to correct. Even after application of the correction, conventional CT does not provide quantitative image values; instead, the same material at different locations often shows different CT numbers.
Another drawback of conventional CT is a lack of material characterization. For example, a highly attenuating material with a low density can result in the same CT number in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object based solely on the CT number.